Some of the decisions in the design of synchronous digital communication systems involve the method of encoding source bits and the method of synchronizing decoding processes in the receiver for correct recovery of the transmitted message.
In a binary data system, each bit received from the channel can be one of two possible symbols. Without a priori knowledge of polarity inversions that the bits may have encountered in crossing the channel, the receiver lacks the information necessary to decode the individual bits. Inversions may occur when the data is modulated by frequency shift keying (FSK) and the signal undergoes frequency translations in being sent across the channel. In FSK modulation, a positive frequency shift of the carrier corresponds to one binary symbol, and a negative shift corresponds to the other. Frequency translations during up- or down-conversions associated with radio transmission or with multiplexing for telephone line transmission may not preserve the meanings of positive and negative frequency modulation shifts.
One of the methods used in the prior art to preserve polarity information across a channel is known as differential encoding. The signal is encoded so that information resides in the difference of polarity from one bit to the next. Using this method avoids the need for a coherent, local phase reference, since each bit serves as the local reference for the succeeding bit. Differential encoding trades noise performance for reduced system complexity. An error in one bit extends to the next, for if an error occurs in one bit, the receiver has an uncertain reference and may not correctly decode the next bit. By degrading error performance, differential encoding may interfere with system operation. In particular, it may diminish the effectiveness of error correction circuits.
The present invention also addresses the method of synchronization used in a communication system. What method is used can affect the ability of the receiver to detect sequences transmitted over the channel for various identification purposes, to maintain or quickly regain synchronization in an noisy channel environment, and to quickly adapt to changes in delay characteristics of the transmission channel. Prior art design choices have frequently involved tradeoffs, in which improvements of one aspect of synchronization performance have meant degradation of another.
An example of transmitting sequences for identification occurs in a synchronous stream encryption system. The transmitter encrypts source bits with a pseudo-random keystream sequence; the receiver decrypts them by an inverse operation. Each channel bit produced depends only on its position in the stream of source bits and on the particular keystream in use. To recover the source bits, the receiver must regenerate the keystream locally and align it with incoming channel bits.
To enable the receiver to regenerate and align the keystream, the transmitter sends an identifying sequence. Not knowing when the sequence will be sent, the receiver must be able to detect the sequence in the presence of random digital data. Detecting a sequence can serve a variety of other purposes. For example, to increase the certainty that the receiver has correctly identified a synchronization or timing mark imbedded within noisy data, the system may require that the receiver have previously detected a unique sequence.
A different aspect of synchronization that the present invention addresses is how the receiver initially establishes synchronization when communication begins and how it maintains synchronization during interruptions of communication. When error correction is used to improve the certainty of synchronization in a noisy environment, initial synchronization is often a slow process. Error correction circuits must process a number of bits to reach full effectiveness. Synchronization attempts made shortly after transmission begins or after recovery from a fade use less certain bits and may, therefore, be less reliable than later attempts, which can use more certain bits.
Delaying synchronization also helps the receiver adapt to changes in delay characteristics of the transmission channel that occur after communication has been established. One type of delay change contemplated by the invention is found in a multiple site system with receivers situated at diverse geographic sites to obtain wide area coverage of the transmitted signal. Each of the receivers sends demodulated output to a central control point that chooses one signal path, according to signal quality, to supply the data decoding circuits. The time scales may be misaligned among the data signals received from the several sites because of propagation differences in the paths from the transmitter to each of the receiving sites or differences in the electrical paths from the remote sites to the control point. Synchronization may be disrupted if the path selection changes after a message commences.
In a typical scenario, the receiver selection occurs shortly after a message begins. The initial selection may be changed before being made final. Synchronization rapidly acquired at the start of the message might later become incorrect if the selection changes. Under these circumstances, it is desirable for the synchronization system to be able to rapidly and automatically readjust.
Delaying synchronization affects the ability of the receiver to quickly recover from momentary communication impairments, so prior art designs have often employed hysteresis to provide adaptive timing. Hysteresis prevents the receiver from attempting to reestablish synchronization, with its attendant delay, if it has been interrupted for only a short time. An example of hysteresis may be found in a communication system designed for a terrestrial radio channel that may be subject to momentary fading. During a fade, the receiver may be unable to continue to recover synchronization information yet have sufficient short-term stability to continue to correctly decode channel bits. Without hysteresis, the receiver would attempt to reestablish synchronization each time it recovered from a fade.
A problem experienced with hysteresis is that it prevents the receiver from responding to synchronization changes until the hysteresis interval has elapsed. This interferes with the ability to respond rapidly, as required in a multiple site receiver system.